An observational study on the relationship between tyrosine kinase inhibitor treatment-free remission and cytotoxic T-lymphocytes in patients with chronic phase chronic myeloid leukemia.

171 Background: Since the introduction of tyrosine kinase inhibitors (TKIs), the effectiveness of treating chronic phase chronic myeloid leukemia has seen remarkable improvement. A current challenge in TKI therapy is achieving treatment-free remission (TFR) safely. Numerous TKI-stop trials and translational studies have investigated factors influencing TFR, including TKI treatment duration, duration of deep molecular response (DMR), cellular immunity activation, and bcr::abl1 transcript type. However, identifying reliable markers for achieving TFR safely remains uncertain. In this multicenter study, we focused on examining the association between TFR and cytotoxic T-lymphocytes (CTLs). Methods: Between June 2020 and March 2022, a total of 45 patients were enrolled: 38 patients with DMR for at least 1 year on TKIs (on TKI group) and 7 patients with DMR for at least 1 year after discontinuing TKIs (off TKI group). The study included 30 males, with a median age of 59 years (range: 29–87). Median TKI treatment duration was 7.7 years (range: 1.5–18.3), and median TKI cessation duration in the off TKI group was 4.9 years (range: 1.7–7.1). Molecular response and cellular immunity were monitored over 12 months post-consent for patients in the off TKI group and after TKI discontinuation for patients in the on TKI group. Results: During the observation period, 25 patients (65.8%) in the on TKI group maintained DMR, while 13 patients (34.2%) lost DMR. Conversely, all patients in the off TKI group sustained DMR. The median TKI treatment duration in the group maintaining DMR (9.45 years, range: 3.3–18.3, N = 32) significantly exceeded that in the group losing DMR (4.9 years, range: 1.5–11.9, N = 13) (P = 0.0048). Patients taking TKIs for over 7 years with a higher percentage of total memory CTLs than total effector CTLs maintained DMR. Conversely, patients losing DMR despite over 10 years of TKI treatment had a higher percentage of total effector CTLs than total memory CTLs. Among the 32 patients maintaining DMR, 19 exhibited a higher percentage of total effector CTLs than total memory CTLs, with 15 (78.9%) maintaining MR5 throughout the study. Furthermore, of the six patients maintaining DMR with relatively short TKI administration (median 5.5 years, range: 3.3–6.4), three displayed a higher percentage of total memory CTLs than total effector CTLs. Among the remaining three patients, two showed strong CTL activation, with one patient having an effector CTL clone over 30% and the other having a memory CTL clone over 30%. Conclusions: This study indicates that a sufficiently prolonged TKI treatment duration (>7 years) and maintaining deeper DMR (≥ MR4.5, preferably MR5) are conducive to safely achieving TFR. Furthermore, a high percentage of total memory CTLs and intense CTL activation may serve as meaningful markers for TFR.